1. Area of the Invention
The present invention relates to a circuit configuration having a feedback operational amplifier for amplifying an input signal input into the circuit configuration and outputting the amplified input signal as an output signal.
Circuit configurations of this type are known in manifold forms. Solely for exemplary purposes, reference is made to the textbooks “U. Tietze, Ch. Schenk, Halbleiter-Schaltungstechnik [Semiconductor Circuit Technology], 5th Edition, Springer-Verlag, 1980”, in particular pages 93 through 108, and “Paul Horwitz, Winfield Hill, The Art of Electronics, Second Edition, Cambridge University Press, 1989”, in particular pages 175 through 195.
The term “operational amplifier” as defined in the present invention is to be understood very broadly as a configuration capable of amplifying an electrical variable such as a voltage. In particular, it refers to amplifiers, for example, in which a signal applied to the amplifier input is provided having relatively high voltage amplification at the amplifier output. This open amplification (“open loop gain”) may be in the magnitude of approximately 104 through 105, for example.
An essential characteristic of the circuit configuration according to the species is that the circuit amplification, i.e., the ratio between output signal and input signal, is practically entirely independent of this open amplification (except for transient occurrences) and is solely predefined by an additional (external) configuration of the operational amplifier.
The term “feedback network” refers in the scope of the present invention to any such configuration of the operational amplifier which exerts an influence on the resulting circuit amplification. In the simplest case, the feedback network comprises a configuration of one or more (ohmic) resistors. Alternatively or additionally, other components such as capacitors and/or inductors may be provided to produce the feedback network. Very generally, these components provided to produce the feedback network are thus identified in the following as impedances.
2. Description of the Prior Art
There are applications in which a circuit configuration is required in which the circuit amplification is continuously changeable. In order to achieve this, essentially two approaches are followed in the prior art.
One possibility is to connect a damping element, whose damping is changeable, upstream of an amplifier having a fixed predefined amplification.
Another possibility is to implement amplification-determining resistors of the feedback network on an operational amplifier by MOS transistors, which are operated in the triode range. The effective resistance of these transistors may be changed continuously by a corresponding change of activation signals (gate source voltages).
Both achievements of the object are not satisfactory in all applications. In the first achievement of the object, nonlinearities typically occur in the damping element, for example, which have an effect on the circuit amplification.
In the second achievement of the object, above all, a disadvantage is the restricted linearity of the resistors implemented by MOS transistors in the triode range and, accompanying this, a reduced linearity of the circuit configuration.